1. Field of the Invention
The invention relates to MEMS devices combined with integrated circuits, particularly as used as biomedical sensors and devices.
2. Description of the Prior Art
Since the emergence of MEMS technology, numerous miniaturized sensors and actuators have been fabricated using techniques originally developed for the integrated circuits industry. Inherently, most MEMS devices are built on rigid substrates. However, for a wide variety of applications, it has long been desirable for sensors, actuators and circuits to be mounted on non-planar surfaces or even on flexible objects such as a human body.
For example, the inventors of the present application have done work on a new way of controlling the Unmanned Aerial Vehicle (UAV) through the sensing/controlling of the flow separation at the leading edge. This requires distributed sensors mounted on the cylindrical surface of leading edge. Accordingly, the inventors have developed flexible shear-stress sensor skins for the UAV project. See F. Jiang et. al., xe2x80x9cFlexible Shear Stress Sensor Skin For Aerodynamics Applications,xe2x80x9d presented at IEEE International Conference on Micro Electro Mechanical Systems (MEMS), Miyazaki, Japan, 2000.
These skins, however, contained only sensors and required many electrical lead connections. The complete separation-detecting system consisted of the packaged sensor skins, bias board, and data acquisition board and a tremendous number of interconnection cables.
In particular, what is needed for a shear-stress sensor skin, with on-skin bias circuits, amplifiers, and multiplexers, is a design that allows elimination of the bias board and interconnection cables, simplification of the design of the data acquisition board, and improvement system reliability at the same time.
Therefore, what is needed is some kind of design that allows integration with these circuits, since this integration promises to bring very important benefits such as operational improvement, packaging simplification and cost reduction.
The invention is directed to a new device called an IC-integrated, flexible, shear-stress sensor skin or MEMS skin. By integrating both bias and signal-conditioning circuitry on-chip, the wiring of the MEMS skin 10 is significantly simplified and reliability is improved. The circuit is first made by a commercial IC foundry and micromachining is done on the CMOS wafers to form the skins. In the illustrated embodiment, the use of the sensor skin is demonstrated by packaging it on a semi-cylindrical aluminum block and tested it in a wind tunnel.
In the illustrated embodiment, the skin has successfully identified both the flow separation and stagnation points. It is believed that the IC integrated skin can be applied to many applications in biomedicine, wearable microsystems, and robotics to mention only a few. The illustrated embodiment may thus be modified according to the intended application without departing from the scope of the invention.
The IC-integrated skin technology demonstrated in this invention has many applications. The first example is for the biomedical applications. Many MEMS devices have already been widely employed in biomedicine where miniaturized sensors, actuators and other microstructures are needed. However, the IC integrated skins advance the art to another level of functionality. The sensor skin can be mounted on human body like a bandage. Examples for future applications may include skins with sensors capable of monitoring physiological parameters such as glucose and insulin levels. For implantable applications, sensors can be built on flexible substrates to conform to the organ shape or to minimize tissue trauma during patient movement. Skins incorporating arrays of tactile, temperature, and other sensors are very helpful to surgical instruments for minimally invasive surgery. The IC-integrated skin is also of great promise for wearable microsystems, robotics and many other research areas. In this invention, Parylene is used as skin material. However, other polymers can also be used to meet the specific requirements of the application.
More specifically, the invention is a method of fabricating an IC-integrated, flexible, shear-stress sensor skin comprising the steps of providing a wafer with integrated circuits and sensor elements coupled thereto, both of which integrated circuits and sensor elements are fabricated in or on the wafer; disposing a first polymer layer on the wafer and on the sensor elements, the first polymer layer providing mechanical support for the sensor elements; defining a cavity below each of the sensor elements to release the sensor elements from the wafer to provide thermal isolation therefrom, while the sensor element remains supported by the first polymer layer; and isolating the sensor elements into a plurality of islands defined in the wafer, so that the islands, at least one with at least one sensor element, and the integrated circuit elements coupled thereto form the IC-integrated, flexible, shear-stress sensor skin.
The step of disposing a first polymer layer on the wafer and on the sensor elements comprises disposing two polymers layers on the wafer and on the sensor elements to form a compound layer, wherein at least one of the two layers is chosen of a material suited for use with the sensor elements.
The step of disposing two polymers layers on the wafer and on the sensor elements to form a compound layer comprises disposing a first of the two polymer layers to form a mechanical support for the islands and disposing a second of the two polymer layers to form a diaphragm support for the sensor elements.
The step of disposing a first of the two polymer layers to form a mechanical support for the islands and disposing a second of the two polymer layers to form a diaphragm support for the sensor elements comprises disposing a Parylene C layer as the first of the two polymer layers to form a mechanical support for the islands and disposing a Parylene N layer as the second of the two polymer layers to form a diaphragm support for the sensor elements.
The first polymer layer is disposed on a first side of the wafer and the method further comprises the step of disposing a second polymer layer on an opposing second side of the wafer to encapsulate the islands between the first and second polymer layers.
The method further comprises the step of thinning the wafer.
The step of isolating the sensor elements into a plurality of islands defined in the wafer comprises defining the islands in the wafer by deep reactive ion etching regions of the wafer containing at least one sensor element.
The step of providing a wafer with integrated circuits and sensor elements coupled thereto comprises providing biasing or signal conditioning circuits adapted for the sensor elements among the integrated circuits provided in the wafer.
The step of providing a wafer with integrated circuits and sensor elements coupled thereto comprises providing a multiplexer for selectively accessing a plurality of the sensor elements on the skin.
The step of disposing a second polymer layer on a side of the wafer to encapsulate the islands between the first and second polymer layers comprises disposing a Parylene C layer as the second polymer layer on the opposing side.
The invention also includes an apparatus or an IC-integrated, flexible, shear-stress sensor skin fabricated by the foregoing methods or otherwise having the resulting structure realized by the fabrication. Namely, the apparatus comprises: a wafer defined into a plurality of islands; at least one integrated circuit fabricated in or on the wafer; at least one sensor element fabricated in or on at least one island and coupled to the integrated circuit; a first polymer layer disposed on the wafer and on the sensor element, the first polymer layer providing mechanical support for the sensor element and islands; and a cavity defined in the island below each sensor element to provide thermal isolation from the island, while the sensor element remains supported by the first polymer layer, so that the islands, each with at least one sensor element, and the integrated circuit coupled thereto form the IC-integrated, flexible, shear-stress sensor skin.
While a shear-stress sensor is described above, the invention is more generally an IC-integrated, flexible sensor skin comprised of a wafer defined into a plurality of islands; at least one integrated circuit fabricated in or on the wafer; with none, one or more sensor elements fabricated in or on each island and coupled to the integrated circuit; a first flexible polymer layer disposed on a first side of the wafer and on the sensor element, the first polymer layer providing mechanical support for islands in at least the case where a shear-stress sensor is employed; a second flexible polymer layer on an opposing second side of the wafer to encapsulate the islands between the first and second polymer layers; and a cavity defined in the island below each sensor element to provide isolation from the island in at least the case where a shear-stress sensor is employed, while the sensor element remains supported by the first polymer layer, so that the islands, with at least one sensor element on at least one island, and the integrated circuit coupled thereto form the IC-integrated, flexible sensor skin. Any parameter may be measured by the sensor and not only shear stresses.
While the apparatus and method has or will be described for the sake of grammatical fluidity with functional explanations, it is to be expressly understood that the claims, unless expressly formulated under 35 USC 112, are not to be construed as necessarily limited in any way by the construction of xe2x80x9cmeansxe2x80x9d or xe2x80x9cstepsxe2x80x9d limitations, but are to be accorded the full scope of the meaning and equivalents of the definition provided by the claims under the judicial doctrine of equivalents, and in the case where the claims are expressly formulated under 35 USC 112 are to be accorded full statutory equivalents under 35 USC 112. The invention can be better visualized by turning now to the following drawings wherein like elements are referenced by like numerals.